Fluvial dynamics and 14C-10Be disequilibrium on the Bolivian Altiplano |
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Authors: | Kristina Hippe Tiemen Gordijn Vincenzo Picotti Irka Hajdas John D Jansen Marcus Christl Christof Vockenhuber Colin Maden Naki Akçar Susan Ivy-Ochs |
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Institution: | 1. Laboratory of Ion Beam Physics, ETH Zürich, 8093 Zürich, Switzerland;2. Geological Institute, ETH Zürich, 8092 Zürich, Switzerland;3. Department of Geoscience, Aarhus University, 8000 Aarhus, Denmark;4. Institute of Geochemistry and Petrology, ETH Zürich, 8092 Zürich, Switzerland;5. Institute of Geological Sciences, University of Bern, 3012 Bern, Switzerland |
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Abstract: | Determining sediment transfer times is key to understanding source-to-sink dynamics and the transmission of environmental signals through the fluvial system. Previous work on the Bolivian Altiplano applied the in situ cosmogenic 14C-10Be-chronometer to river sands and proposed sediment storage times of ~10–20 kyr in four catchments southeast of Lake Titicaca. However, the fidelity of those results hinges upon isotopic steady-state within sediment supplied from the source area. With the aim of independently quantifying sediment storage times and testing the 14C-10Be steady-state assumption, we dated sediment storage units within one of the previously investigated catchments using radiocarbon dating, cosmogenic 10Be-26Al isochron burial dating, and 10Be-26Al depth-profile dating. Palaeosurfaces appear to preserve remnants of a former fluvial system, which has undergone drainage reversal, reduction in catchment area, and local isostatic uplift since ~2.8 Ma. From alluvium mantling the palaeosurfaces we gained a deposition age of ~580 ka, while lower down fluvial terraces yielded ≤34 ka, and floodplains ~3–1 ka. Owing to restricted channel connectivity with the terraces and palaeosurfaces, the main source of channel sediment is via reworking of the late Holocene floodplain. Yet modelling a set of feasible scenarios reveals that floodplain storage and burial depth are incompatible with the 14C-10Be disequilibrium measured in the channel. Instead we propose that the 14C-10Be offset results from: (i) non-uniform erosion whereby deep gullies supply hillslope-derived debris; and/or (ii) holocene landscape transience associated with climate or human impact. The reliability of the 14C-10Be chronometer vitally depends upon careful evaluation of sources of isotopic disequilibrium in a wide range of depositional and erosional landforms in the landscape. © 2018 John Wiley & Sons, Ltd. |
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Keywords: | cosmogenic nuclide dating steady-state 14C-10Be chronometer sediment storage radiocarbon dating |
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